The present invention relates in general to integrated circuits, and more particularly to buffering circuitry with slew rate control for use in integrated circuits.
Buffer circuits are very common circuits used throughout integrated circuit design. Specific elements required by a buffer circuit may vary depending upon the constraints with which the buffer circuit must function. Although buffer circuits can be used throughout an integrated circuit, one common place where they are normally used is as an input/output buffer at the integrated circuit pads which are used to couple the integrated circuit to the external world.
One set of constraints for an input/output buffer is provided by the IEEE 1284 standard, which is used for driving a 62 ohm transmission line from the integrated circuit. The 1284 IEEE standard requires a precise slew rate, a precise output impedance, as well as input hysteresis. In addition to the constraints required by the 1284 IEEE standard, it is also often a requirement that an integrated circuit use as little power as possible. In addition, it is also desirable for most integrated circuits to have electrostatic discharge (ESD) protection. As yet another constraint, the internal circuitry within an integrated circuit may be designed using a process that requires a lower power voltage than the voltage required by the IEEE 1284 standard which requires 5 volts.
Although a buffer may be designed to meet a variety of design constraints, particular portions of the buffer design may be applicable to a wide variety of usages, some of which are outside of particular sets of constraints. For example, particular portions of a buffer design that improve ESD protection may be applicable to a wide variety of buffer circuits that do not have similar constraints. In addition, a buffer circuit that is designed to meet the IEEE 1284 standard for slew rate may be applicable to buffers that are used in other applications not requiring that standard. In addition, there may be particular value to subsets of the constraints listed above. For example, it may be particularly useful to have a buffer circuit that is both low power and meets the IEEE 1284 standard.